My grandmother made quilts from the late 1890's till she died in 1960's. I never saw her buy fabric. She always recycled before it was called that.
So I, in a pinch, have at times followed in her footsteps. Like when I used an old electric blanket for batting (after I pulled all the wires out). Boy was that a nice warm quilt afterwards! My kids used to fight over it. I had to tie that one, not quilt it.
I've also used fleece between the top and backing, eventhough I know the fleece could have been used AS the backing.
I also remember using old flannel flat sheets (2 at a time).
What have you used?